I agree with you that the increased lift is likely to be minimal, unless of course you throw form out of the window and you're doing all you can do get that bar back up. It's something a lot of people "state". I already stated that it was a poor choice of words "weakening core" I meant to state, as I already have, that owing to a weaker core a lot of people use belts, and when they lift, they get away with poor form, as lifting with poor form helps a belt's usage. If you lift with poor form, you get even more out of a belt - as such, it enhances poor form when using a belt exclusively. Now I know this doesn't apply to 99% of people in this thread and on OcUK, but it's important to note that belts, aren't necessary for most people as good form and a strong core should be achievable without it. I do concede that if you've had lower back issues either through previous injury and so on, then yes using a belt is sensible.
Ultimately, you've said the right thing, they key is good form irrespective of a belt. I fully understand the biomechanics of increasing pressure in the abdominal area, however, in general, unless on the limit of your capacity they do add little - and I question whether or not you should have to use a belt if you're getting near your limit at the detriment of form and core strength.